This invention relates to a polyolefin composition for cellular insulation that has improved capability to be processed at high line speeds with enhanced foamability.
Thin wall insulated wires incorporating foamed polyolefin insulating materials are commonly used in local area network (LAN), wiring and outdoor xe2x80x9ctelephonexe2x80x9d cables. These insulated wires are typically produced using a wire coating extrusion process operating at production speeds ranging from about 500 to about 3000 meters per minute. Increased production speeds are desired for increased productivity resulting in improved production economics, but can be limited by the capability of the insulation material to be extruded as a very uniform foamed insulation layer within physical and electrical production tolerances. Very uniform cellular insulation is especially important to data-grade transmission characteristics, which are commercially evolving toward increasingly stringent requirements. Productivity can also be improved by increasing the foaming capability of the insulation to provide higher expansion rates, allowing reduced insulation thickness and material use.
Foaming can be accomplished by chemical foaming agents or by physical gas injection into the extruder during the high speed insulated wire production process. The insulated wire can be a single foamed insulation layer, or can be a multi-layer design such as the insulation widely used in telephone cables, which has a foamed inner skin and a solid outer skin. The foamed polyethylene insulation provides some improved electrical properties, for example, higher velocity of propagation and/or lower attenuation, versus solid (not foamed) polyolefin insulation. Foaming also improves cable economics via reduced insulation weight and decreased insulation thickness yielding decreased cable dimensions.
Thin wall cellular insulated wire for LAN or telephone cable use is typically twisted in a helical manner with another similar insulated wire to form a xe2x80x9ctwisted pairxe2x80x9d transmission circuit. The insulated wires are usually colored to provide color coded transmission pairs that facilitate installation into communication networks. Multiple transmission pairs are usually grouped together, often by additional helical twisting, to form a transmission core. The transmission core is then protected by a sheathing system which incorporates a polymeric jacketing to provide mechanical and environmental protection. Often metallic armor and/or multiple jacketing layers are included in the sheathing system for added environmental protection, especially for outdoor cables. With outdoor cables, hydrocarbon greases are often used to fill air spaces in the cable, such as the interstices between the twisted pairs in the transmission core, to minimize the possibility of deterioration in electrical transmission performance by water/moisture ingress. Various other components such as polymeric wraps or metallized shields for electromagnetic interference (EMI) protection and flame retardancy are often incorporated into the cable design, especially for LAN cable applications. For indoor applications such as LAN cables, the cable usually must meet certain flame retardancy standards, which can affect material selection and cable design.
The selection of the polyolefin insulating material(s) is a critical factor in determining the production speeds and end-use performance capabilities for high speed thin wall cellular insulation applications. The best available commercial materials have limited production capabilities such that improved materials are very much desired. For example, existing commercial materials used in a physical foaming process for production of thin wall foamed insulated wires for LAN data-grade applications typically lack good high speed extrusion characteristics. Therefore, production speeds are typically limited to about 1500 meters per minute or less. Attempted production above such speeds typically results in insulation roughness with increased capacitance and diameter variations, yielding unacceptable electrical transmission characteristics. Despite production speed limitations, these resins have been used commercially for LAN applications due to good foamability that allows for expansion rates up to 65 percent. Until the present invention, other resins having better high speed extrusion capability lacked the desired foaming capability. Another example is the multilayer foam/skin telephone wire application in which chemically expanded foams are typically used. Commercial materials traditionally used in this application have good high speed extrusion characteristics but lack optimal foaming capability. Such materials allow for foam/skin production at speeds up to about 3000 meters per minute or more but their limited foaming capability restricts use to about a 45 percent maximum expansion level. Attempts to foam these materials above about a 45 percent expansion yields unacceptable foam quality and insulation variations that exceed allowable tolerances for acceptable electrical transmission characteristics. Due to the ability of processing at high production rates, existing commercial materials have been used despite their limited foaming capability.
Thus a base resin system providing a combination of improved high speed extrusion capability and improved foaming capability will provide substantial product improvement. Such an improved composition for cellular insulation would enable increased production speeds for, for example, gas injection foamed LAN insulation, and increased foaming capability for, for example, chemically foamed telephone wire insulation. This would improve productivity via increased output rates and increased expansion rates providing reduced material consumption.
The production of a chemically foamed insulation typically comprises the following steps. A chemical blowing agent is melt mixed, or compounded, with the base resin system at a temperature below the decomposition temperature of the selected blowing agent. The melt is then pumped through a pelleting dieplate and a pelleter/cutter to produce small pellets (solid beads) which are then cooled to solidification temperature in water. This pelleted composition is then used as the raw material for the foamed insulation extrusion process. It is also possible to obtain a chemical foam insulation composition by combining several constituents in the proper ratios at the fabrication extruder. For example, a pellet mixture comprising the proper ratio of base resin components and a chemical blowing agent masterbatch could be dry blended and then fed directly to the fabrication extruder.
To fabricate the foamed insulation, the specified composition is typically processed in a fabricating extruder for coating onto a conductor. In the fabricating extruder, the pellets are fluxed and mixed and the melt is then brought to a processing temperature exceeding the decomposition temperature of the blowing agent, thereby producing gas for the foaming process. As the melt passes through the coating die, forming a coating around the wire to be insulated, the dissolved gas nucleates and forms tiny cells in the plastic coating. This foaming mainly occurs in an air gap between the die exit and a water cooling trough, in which the insulation is solidified. Chemically foamed insulation is widely used because the required equipment investment is lower and because the operation has competitive processing capabilities relative to the physical foaming process, at lower expansion rates, especially for foam/skin telephone wire insulation production.
For high speed foam/skin telephone wire applications, commercially available chemically expanded foam materials have good cellular insulation processing capability to about 45 percent expansion. As noted above, typical foam/skin production line speeds range from 1500 to 3000 meters per minute. For other chemically foamed insulation processes, notably coaxial cables which are produced to larger diameters at considerably slower production speeds (less than approximately 200 meters per minute), expansion rates of up to 60 percent have been achieved. New chemical foam compositions are being developed which will boost expansion rates to about 70 percent for the larger diameter coaxial cable foam applications, but these compositions fail to have the capability for processing at high line speeds and have very delicate processing characteristics.
The expansion limit of an insulation material is determined both by material and extrusion process capabilities to produce a foamed insulation with good cell uniformity and dimensional control. As previously noted, good cellular insulation uniformity is needed to maintain electrical transmission characteristics within tight target ranges.
Physical foaming processes use a gas that is physically pumped into the polymer melt in the barrel of the fabricating extruder instead of being generated in situ with a chemical blowing agent. Chlorofluorocarbon gases such as monofluorotrichloromethane, difluorodichloromethane, trifluorotrichloroethane, and tetrafluorodichloroethane were historically used in gas injection processes to obtain highly foamed insulation with 80 percent (by volume) expansion or more. Chlorofluorocarbon gases, however, are being phased out because of their negative environmental impact. Other gases such as argon, carbon dioxide and especially nitrogen are now typically used and show somewhat increased difficulty in achieving a uniform fine foaming structure in the fabricated insulation. The conversion from chlorofluorocarbon gases to inert gases requires screw design modifications in order to achieve comparable expansion rates due to the lower solubility of these gases in polyolefins such as polyethylene. Production speed capabilities will vary over a considerable range depending upon materials used, the equipment and operating conditions used, the insulation design (multiple or single layers) and allowed production tolerances. For larger diameter coaxial cable compounds, gas injection foaming to 80 percent expansion is commercially practiced with relatively low line speeds (less than 500 meters per minute). On the other hand, production speeds up to 1500 meters per minute are possible in thin wall cellular insulation applications where expansion levels are reduced, typically to about a 50 percent expansion level. The production rates and expansion levels for thin wall cellular insulation by the gas injection process are limited by the availability of commercial polyolefin materials having both good foaming capability and good high speed extrusion characteristics.
An object of this invention, therefore, is to provide a resin composition that can be extruded at high speeds and exhibits an enhanced foaming capability, i.e., high expansion, for thin wall cellular insulation by the use of chemical or physical foaming. Other objects and advantages will become apparent hereinafter.
According to the present invention, the object is met by the following expandable resin composition.
The composition comprises:
(A) about 60 to about 98 percent by weight of a polyolefin or mixture of polyolefins wherein the polyolefin or mixture has an xcex70 of less than about 9.0 kilopascal.seconds (kPa.s), a Jr greater than about 50xc3x9710xe2x88x925/pascals (Pa), and an Ea greater than about 6.7 kilocalories per mole (kcal/mol); and
(B) about 2 to about 40 percent by weight of a polyolefin or mixture of polyolefins having an nRSI greater than about 4.5, and no greater than about 19; or
(C) a polyolefin or mixture of polyolefins wherein the polymer or mixture has an xcex70 of less than about 9.0 kilopascal.seconds, a Jr greater than about 50xc3x9710xe2x88x925/pascals, an Ea greater than about 6.7 kilocalories/mole, and an nRSI greater than about 9.
In a preferred embodiment, component (A) and component (B) are present in the composition at levels of about 75 to about 98 percent by weight and about 2 to about 25 percent by weight, respectively. In a most preferred embodiment, component (A) and component (B) are present in the composition at levels of about 88 to about 98 percent by weight and about 2 to about 12 percent by weight, respectively.
Component (A) is such that, under processing conditions, bubbles comprising the developing foam and generated by chemical or physical means can grow easily in the polymer melt without significant coalescence. Ease of bubble growth and resistance to bubble coalescence are attained by a low zero-shear viscosity xcex70 and high recoverable compliance (Jr), respectively. It is understood by those familiar with rheological methods that resistance to bubble coalescence is also often characterized by high melt strength or, in other words, a molecular structure that leads to high flow activation energy (Ea).
Component (B) is a shear thinning agent such that extrusion performance and surface appearance of the composition are enhanced relative to component (A) alone without detrimentally affecting foamability. The ability to enhance extrusion performance is characterized by a high value for the normalized relaxation spectrum index (nRSI). Materials other than component (B) or component (B) at higher or lower fractions of the overall composition will fail to enhance extrusion performance without detrimentally affecting foamability.
With respect to components (A) and (B):
xcex70 is preferably less than about 7.0 kilopascal.seconds, and most preferably less than about 5.0 kilopascal.seconds.
Jr is preferably greater than about 60xc3x9710xe2x88x925/pascals, and most preferably greater than about 70xc3x9710xe2x88x925/pascals.
nRSI is preferably greater than about 8, and most preferably greater than about 9.
Component (C) is a composite of components (A) and (B), though not necessarily a mixture of more than one polyolefin. In this case, rheological measurements are made on the composite rather than on component (A) or component (B) . Preferred ranges are as above for xcex70, Jr, and Ea. nRSI, however, is preferably greater than about 9.5 and, most preferably, greater than about 10.
In a practical sense, for components (A) and (B) or component (C), xcex70 is preferably no less than about 0.1 kilopascal.seconds, Jr is preferably no greater than about 150xc3x9710xe2x88x925/pascals, and Ea is preferably no greater than about 15 kilocalories/mole.
The described composition, particularly in its preferred and most preferred embodiments, will have the desired balance of high speed extrudability and enhanced foamability, regardless of whether the polymeric material has been formed in a single or multiple reactor process, with one or more catalysts or catalyst systems, as an in situ blend or with post-reactor processing, or by gas phase or other polymerization process such as liquid phase or in solution.
The xcex70, Jr, nRSI, and Ea of a polymeric material are simultaneously or separately determined by first subjecting the material to a shear deformation, and then measuring its response to the deformation using a rheometer.
The xcex70 and Jr of a polymeric material are calculated from the measured strain during the application of or recovery from an applied constant stress. During application of a constant stress, a polymeric material will increase its strain at a rapid rate until eventually assuming steady state behavior during which the rate of strain increase is constant. The xcex70 is calculated as the applied stress divided by the rate of change of the time-dependent strain during steady state behavior, or
xcex70="sgr"/(xcex4xcex3/xcex4t)ss
where "sgr" is the applied stress. The Jr of a polymeric material is calculated either from an extrapolation of the steady state time-dependent strain data to zero time, or preferably from the asymptotic value of the extent of strain recovery following relaxation of the applied stress, or
xe2x80x83Jr=[xcex3r(0)xe2x88x92xcex3r(t)]/ "sgr" (as t approaches infinity)
where xcex3r(0) and xcex3r(t) refer to the measured strain at the start of and at some time during recovery from the applied stress, respectively.
For xcex70 and Jr, there are blending rules found in the literature that are based, to some extent, on molecular theory. For a two-component blend [component 2 having a higher molecular weight (MW) than component 1], they are:
log(xcex70b)=a log(w1xcex7011/a+w2xcex7021/a)
where w1 and w2 are the respective weight fractions in the blend for component 1 and component 2 respectively, a is the exponent in the well known viscosity-MW relationship, with a value usually in the range of 3.4 to 3.6, and
Jrb=w1Jr1(l1xcex701/xcex70b)+w2Jr2(l2xcex702/xcex70b)
where l1 and l2 are parameters related to the shift of a characteristic longest relaxation time (xcexi) of the respective component in the blend relative to that of a pure component       l    i    =                    λ                  i          ,          b                            λ                  i          ,          pure                      .  
The characteristic longest relaxation time (xcexi) is defined as in the literature. In the above, parameters with the xe2x80x9cbxe2x80x9d subscript refer to those for the blend, while those with a numerical subscript refer to those of the pure components (note that xcex70 for the blend is required for the calculation of Jr for the blend). If one assumes that the relaxation times for the components in the blend are the same as the relaxation times as pure components, all of the l-values are 1.0. In a blend of a high and a low MW component, since the relaxation time of a high MW component is generally less than that in its pure component state, and since the relaxation time of a low MW component is generally higher, l1 tends to be greater than 1, while l2 tends to be less than 1.
The key to predicting Jr for the blends of interest is to find a rigorous method for estimating the time shift factors (l1 and l2) for any combination of components. For low MW polymers, the following relationship is proposed:                               l          i                =                                            η              b                        ⁢                          M              i                                                          η              i                        ⁢                                          M                _                            w                                                          (        1        )            
which becomes                               l          i                =                              (                                          η                b                                            η                i                                      )                                              (                              a                -                1                            )                        /            a                                              (        2        )            
after substituting for the viscosities and then the molecular weights of the blend and the pure component using the well known viscosity-MW relationship
xcex70=KMa
where a is approximately 3.6 for polyethylene. Equation (1) relates the shift in relaxation time upon blending to the ratio of the component viscosity to that of the blend and though it is based on the rheological behavior of low MW polymers, it provides a reasonable and general method for estimating li values for polymers of higher MW. Equation (2) was used to calculate the time shift parameters for all of the examples mentioned below.
Another rheological indicator used to define the specific material properties of the present invention is the flow activation energy, Ea . Ea is calculated from dynamic oscillatory shear data collected on the same sample but at different experimental temperatures. The shift in the experimental data along the frequency axis relative to the experimental data at some reference temperature, typically 190 degrees C., is calculated. The set of temperature shift, aT, and temperature data are then fit to an Arrhenius expression,       a    T    =      exp    ⁡          [                                    E            a                    R                ⁢                  xe2x80x83                ⁢                  (                                    1                              T                +                273.15                                      -                          1                                                T                  0                                +                273.15                                              )                    ]      
where T and T0 are the experimental and reference temperatures, respectively, in degrees C, and R is the ideal gas constant (see Dealy et al, Melt Rheology and Its Role in Plastics Processing, Van Nostrand Reinhold, 1990, page 383). Ea is calculated from the fit of the temperature shift and temperature data to the above expression.
As is known in the art, based on the response of the polymer and the mechanics and geometry of the rheometer used, the relaxation modulus G(t) or the dynamic moduli Gxe2x80x2(w) and Gxe2x80x3(w) can be determined as functions of time t or frequency w, respectively (See Dealy et al, Melt Rheology and Its Role in Plastics Processing, Van Nostrand Reinhold, 1990, pages 269 to 297). The mathematical connection between the dynamic and storage moduli is a Fourier transform integral relation, but one set of data can also be calculated from the other using the well known relaxation spectrum (See Wasserman, J. Rheology, Vol. 39, 1995, pages 601 to 625). Using a classical mechanical model, a discrete relaxation spectrum consisting of a series of relaxations or xe2x80x9cmodesxe2x80x9d, each with a characteristic intensity or xe2x80x9cweightxe2x80x9d and relaxation time, can be defined. Using such a spectrum, the moduli are re-expressed as:   "AutoLeftMatch"                                                        G              xe2x80x2                        ⁢                          xe2x80x83                        ⁢                          (              ω              )                                =                                    ∑                              i                =                1                            N                        ⁢                          xe2x80x83                        ⁢                                          g                i                            ⁢                              xe2x80x83                            ⁢                                                                    (                                          ω                      ⁢                                              xe2x80x83                                            ⁢                                              λ                        i                                                              )                                    2                                                  1                  +                                                            (                                              ω                        ⁢                                                  xe2x80x83                                                ⁢                                                  λ                          i                                                                    )                                        2                                                                                                                                              G              xe2x80x3                        ⁡                          (              ω              )                                =                                    ∑                              i                =                1                            N                        ⁢                          xe2x80x83                        ⁢                                          g                i                            ⁢                              xe2x80x83                            ⁢                                                ω                  ⁢                                      xe2x80x83                                    ⁢                                      λ                    i                                                                    1                  +                                                            (                                              ω                        ⁢                                                  xe2x80x83                                                ⁢                                                  λ                          i                                                                    )                                        2                                                                                                                                G            ⁢                          xe2x80x83                        ⁢                          (              t              )                                =                                    ∑                              i                =                1                            N                        ⁢                          xe2x80x83                        ⁢                                          g                i                            ⁢                              xe2x80x83                            ⁢              exp              ⁢                              xe2x80x83                            ⁢                              (                                                      -                    t                                    /                                      λ                    i                                                  )                                                        
where N is the number of modes and gi and xcexi are the weight and time for each of the modes (See Ferry, Viscoelastic Properties of Polymers, John Wiley and Sons, 1980, pages 224 to 263). A relaxation spectrum may be defined for the polymer using software such as IRIS(trademark) Theological software, which is commercially available from IRIS(trademark) Development. Once the distribution of modes in the relaxation spectrum is calculated, the first and second moments of the distribution, which are analogous to Mn and Mw, the first and second moments of the molecular weight distribution, are calculated as follows:   "AutoLeftMatch"                                          g            I                    =                                    ∑                              i                =                1                            N                        ⁢                          xe2x80x83                        ⁢                                          g                i                            /                                                ∑                                      i                    =                    1                                    N                                ⁢                                  xe2x80x83                                ⁢                                                      g                    i                                    /                                      λ                    i                                                                                                                                g            II                    =                                    ∑                              i                =                1                            N                        ⁢                          xe2x80x83                        ⁢                                          g                i                            ⁢                              xe2x80x83                            ⁢                                                λ                  i                                /                                                      ∑                                          i                      =                      1                                        N                                    ⁢                                      xe2x80x83                                    ⁢                                      g                    i                                                                                          
RSI is defined as gII/gI. Further, nRSI is calculated from RSI as described in U.S. Pat. No. 5,998,558, according to
nRSI=RSIxc3x97MI{circumflex over ( )}a
where a is approximately 0.6. The nRSI is effectively the RSI normalized to an MI of 1.0, which allows comparison of rheological data for polymeric materials of varying MIs. Because RSI and nRSI are sensitive to such parameters as a polymer""s molecular weight distribution, molecular weight, and long chain branching, it is a sensitive and reliable indicator of the stress relaxation, as well as the shear thinning behavior, of the polymer. The higher the value of nRSI, the broader the relaxation time distribution of the polymer, therefore the greater its ability to perform as a shear thinning agent.
Polyolefin, as that term is used herein, is a thermoplastic resin. Each polyolefin or mixture of polyolefins is selected according to the parameters defined above. The polyolefin can be a homopolymer or a copolymer produced from two or more comonomers, or a blend of two or more of these polymers, conventionally used as jacketing and/or insulating materials in wire and cable applications. The polymers can be crystalline, amorphous, or combinations thereof. They can also be block or random copolymers. The monomers useful in the production of these homopolymers and copolymers can have 2 to 20 carbon atoms, and preferably have 2 to 12 carbon atoms. Examples of these monomers are alpha-olefins such as ethylene, propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene; unsaturated esters such as vinyl acetate, ethyl acrylate, methyl acrylate, methyl methacrylate, t-butyl acrylate, n-butyl acrylate, n-butyl methacrylate, 2-ethylhexyl acrylate, and other alkyl acrylates; diolefins such as 1,4-pentadiene, 1,3-hexadiene, 1,5-hexadiene, 1,4-octadiene, and ethylidene norbornene, commonly the third monomer in a terpolymer; other monomers such as styrene, p-methyl styrene, alpha-methyl styrene, p-chloro styrene, vinyl naphthalene, and similar aryl olefins; nitrites such as acrylonitrile, methacrylonitrile, and alpha-chloroacry-lonitrile; vinyl methyl ketone, vinyl methyl ether, vinylidene chloride, maleic anhydride, vinyl chloride, vinylidene chloride, vinyl alcohol, tetrafluoroethylene, and chlorotri-fluoroethylene; and acrylic acid, methacrylic acid, and other similar unsaturated acids.
The homopolymers and copolymers referred to can be non-halogenated, or halogenated in a conventional manner, generally with chlorine or bromine. Examples of halogenated polymers are polyvinyl chloride, polyvinylidene chloride, and polytetra-fluoroethylene. The homopolymers and copolymers of ethylene and propylene are preferred, both in the non-halogenated and halogenated form. Included in this preferred group are terpolymers such as ethylene/propylene/diene monomer rubbers.
With respect to polypropylene: homopolymers and copolymers of propylene and one or more other alpha-olefins wherein the portion of the copolymer based on propylene is at least about 60 percent by weight based on the weight of the copolymer can be used to provide the polyolefin of the invention. Polypropylene can be prepared by conventional processes such as the process described in U.S. Pat. No. 4,414,132. Preferred polypropylene alpha-olefin comonomers are those having 2 or 4 to 12 carbon atoms.
Polyethylene, as that term is used herein, is a homopolymer of ethylene or a copolymer of ethylene and a minor proportion of one or more alpha-olefins having 3 to 12 carbon atoms, and preferably 4 to 8 carbon atoms, and, optionally, a diene, or a mixture or blend of such homopolymers and copolymers. The mixture can be a mechanical blend or an in situ blend. Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The polyethylene can also be a copolymer of ethylene and an unsaturated ester such as a vinyl ester, e.g., vinyl acetate or an acrylic or methacrylic acid ester.
The polyethylene can be homogeneous or heterogeneous. The homogeneous polyethylenes usually have a polydispersity (Mw/Mn) in the range of about 1.5 to about 3.5 and an essentially uniform comonomer distribution, and are characterized by single and relatively low DSC melting points. The heterogeneous polyethylenes, on the other hand, have a polydispersity (Mw/Mn) greater than 3.5 and do not have a uniform comonomer distribution. Mw is defined as weight average molecular weight and Mn is defined as number average molecular weight. The polyethylenes can have a density in the range of 0.860 to 0.965 gram per cubic centimeter. They also can have a melt index in the range of about 0.1 to greater than about 100 grams per 10 minutes. A granular polyethylene is preferable.
The polyethylenes can be produced by low or high pressure processes. They are preferably produced in the gas phase, but they can also be produced in the liquid phase in solutions or slurries by conventional techniques. Low pressure processes are typically run at pressures below 1000 psi whereas high pressure processes are typically run at pressures above 15,000 psi.
Typical catalyst systems, which can be used to prepare these polyethylenes, are magnesium/titanium based catalyst systems, which can be exemplified by the catalyst system described in U.S. Pat. No. 4,302,565 (heterogeneous polyethylenes); vanadium based catalyst systems such as those described in U.S. Pat. No. 4,508,842 (heterogeneous polyethylenes) and U.S. Pat. Nos. 5,332,793; 5,342,907; and 5,410,003 (homogeneous polyethylenes); a chromium based catalyst system such as that described in U.S. Pat. No. 4,101,445; a metallocene catalyst system such as those described in U.S. Pat. Nos 4,937,299, 5,272,236, 5,278,272, and 5,317,036 (homogeneous polyethylenes); or other transition metal catalyst systems. Many of these catalyst systems are often referred to as Ziegler-Natta catalyst systems or Phillips catalyst systems. Catalyst systems, which use chromium or molybdenum oxides on silica-alumina supports, can be included here. Typical processes for preparing the polyethylenes are also described in the aforementioned patents. Typical in situ polyethylene blends and processes and catalyst systems for providing same are described in U.S. Pat. Nos. 5,371,145 and 5,405,901. The various polyethylenes can include low density homopolymers of ethylene made by high pressure processes (HP-LDPEs), linear low density polyethylenes (LLDPEs), very low density polyethylenes (VLDPEs), medium density polyethylenes (MDPEs), high density polyethylene (HDPE) having a density greater than 0.940 gram per cubic centimeter, and metallocene copolymers with various densities. The latter five polyethylenes are generally made by low pressure processes. A conventional high pressure process is described in Introduction to Polymer Chemistry, Stille, Wiley and Sons, New York, 1962, pages 149 to 151. The high pressure processes are typically free radical initiated polymerizations conducted in a tubular reactor or a stirred autoclave. In the stirred autoclave, the pressure is in the range of about 10,000 to 30,000 psi and the temperature is in the range of about 175 to about 250 degrees C., and in the tubular reactor, the pressure is in the range of about 25,000 to about 45,000 psi and the temperature is in the range of about 200 to about 350 degrees C.
Copolymers comprised of ethylene and unsaturated esters are well known, and can be prepared by the conventional high pressure techniques described above. The unsaturated esters can be alkyl acrylates, alkyl methacrylates, and vinyl carboxylates. The alkyl group can have 1 to 8 carbon atoms and preferably has 1 to 4 carbon atoms. The carboxylate group can have 2 to 8 carbon atoms and preferably has 2 to 5 carbon atoms, The portion of the copolymer attributed to the ester comonomer can be in the range of about 5 to about 50 percent by weight based on the weight of the copolymer, and is preferably in the range of about 15 to about 40 percent by weight. Examples of the acrylates and methacrylates are ethyl acrylate, methyl acrylate, methyl methacrylate, t-butyl acrylate, n-butyl acrylate, n-butyl methacrylate, and 2-ethylhexyl acrylate. Examples of the vinyl carboxylates are vinyl acetate, vinyl propionate, and vinyl butanoate. The melt index of the ethylene/unsaturated ester copolymers can be in the range of about 0.5 to about 50 grams per 10 minutes, and is preferably in the range of about 2 to about 25 grams per 10 minutes. One process for the preparation of a copolymer of ethylene and an unsaturated ester is described in U.S. Pat. No. 3,334,081.
The VLDPE can be a copolymer of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms and preferably 3 to 8 carbon atoms. The density of the VLDPE can be in the range of 0.870 to 0.915 gram per cubic centimeter. It can be produced, for example, in the presence of (i) a catalyst containing chromium and titanium, (ii) a catalyst containing magnesium, titanium, a halogen, and an electron donor; or (iii) a catalyst containing vanadium, an electron donor, an alkyl aluminum halide modifier, and a halocarbon promoter. Catalysts and processes for making the VLDPE are described, respectively, in U.S. Pat. Nos. 4,101,445; 4,302,565; and 4,508,842. The melt index of the VLDPE can be in the range of about 0.1 to about 20 grams per 10 minutes and is preferably in the range of about 0.3 to about 5 grams per 10 minutes. The portion of the VLDPE attributed to the comonomer(s), other than ethylene, can be in the range of about 1 to about 49 percent by weight based on the weight of the copolymer and is preferably in the range of about 15 to about 40 percent by weight. A third comonomer can be included, e.g., another alpha-olefin or a diene such as ethylidene norbornene, butadiene, 1,4-hexadiene, or a dicyclopentadiene. Ethylene/propylene copolymers and ethylene/propylene/diene terpolymers are generally referred to as EPRs and the terpolymer is generally referred to as an EPDM. The third comonomer can be present in an amount of about 1 to 15 percent by weight based on the weight of the copolymer and is preferably present in an amount of about 1 to about 10 percent by weight. It is preferred that the copolymer contain two or three comonomers inclusive of ethylene.
The LLDPE can include the VLDPE and MDPE, which are also linear, but, generally, has a density in the range of 0.916 to 0.925 gram per cubic centimeter. It can be a copolymer of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms, and preferably 3 to 8 carbon atoms. The melt index can be in the range of about 0.2 to about 180 grams per 10 minutes, and is preferably in the range of about 0.5 to about 5 grams per 10 minutes. The HDPE can include both homopolymers and copolymers of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms and preferably 3 to 8 carbon atoms. The density of the HDPE can include MDPE, but is generally in the range of 0.940 to 0.965 gram per cubic centimeter. The melt index can be in the range of about 0.1 to about 160 grams per 10 minutes, and is preferably in the range of about 0.1 to about 8 grams per 10 minutes. The alpha-olefins for LLDPE and HDPE can be the same as those mentioned above, and the catalysts and processes are also the same subject to variations necessary to obtain the desired densities and melt indices.
As noted, included in the definition of polyethylene are homopolymers of ethylene made by a conventional high pressure process. The homopolymer preferably has a density in the range of 0.910 to 0.930 gram per cubic centimeter. The homopolymer can also have a melt index in the range of about 0.1 to about 45 grams per 10 minutes, and preferably has a melt index in the range of about 0.1 to about 5 grams per 10 minutes. Melt index is determined under ASTM D-1238, Condition E. It is measured at 190 degrees C. and 2160 grams.
A mixture of components (A) and (B) or component (C) together with selected additives is thoroughly melt mixed or compounded in conventional plastics compounding equipment and is then pelletized for use in insulation fabrication equipment. Such melt processing is typically performed at a temperature in the range of about 150 to about 200 degrees C. Melt compounding equipment includes single stage mixers that both melt, mix and pump the material through a pelleting die system such as twin screw compounders or single screw mixing extruders. Also included are two stage continuous and batch mixing systems, in which melting and most of the mixing are accomplished in a fluxing/mixing unit before discharging into a melt pumping device. Two stage continuous mixers include equipment as the Farrel(trademark) FCM or Japan Steel Works(trademark) JSW, while an example of a batch mixer is the Farrel Banbury(trademark) type. Alternatively, the proper ratios of one or more constituents is combined at the fabricating extruder, typically by metering feeds of pelleted materials or by utilizing a dry blend of the constituent materials. The term xe2x80x9cexpandable resin compositionxe2x80x9d means that, in addition to components (A) and (B) or (C), the composition is such that, under physical or chemical foaming conditions, the resin composition will expand or foam.
For polyolefin applications, typical melt processing temperatures during the insulation extrusion process are in the range of about 140 to about 230 degrees C. The media to be coated with foamed insulation can be a metallic electrical conductor or optic fiber, or a core containing two or more of same, or possibly other substrates on which a foamed coating is desired. This media is typically traveling perpendicular to the fabricating extruder through a coating crosshead die assembly located at the discharge end of the fabricating extruder. The coating crosshead turns the melt flow and creates a flow surrounding the media to be coated, with a coating die utilized for the final shaping of the melt. The gases in the polymer melt foam at the lower pressures in the coating die and especially in the air gap between the die exit and the cooling troughs. Water is typically used as the coolant in the cooling troughs used to solidify the foamed insulation on, for example, metallic conductors, or other media. As previously outlined, gas injection foaming is used to obtain cellular insulations with expansion levels of 80 percent or more by volume.
In a physical foaming process, the expandable resin composition is continuously fed into an extruder adapted to melt the resin and simultaneously introduce an inert gas that is injected into the melt under pressure. The inert gas, for example nitrogen, is introduced to the extruder in an amount of about 0.01 to about 10 parts by weight of inert gas per 100 parts by weight of expandable resin composition. Other inert gases that can be use include helium, neon, krypton, xenon, radon, and carbon dioxide. Nitrogen and carbon dioxide are preferred. An optimal extrusion configuration provides sufficient mixing and residence time for the gas to uniformly dissolve into the melt. As the melt is coated onto the wire and exits the high pressure extruder environment through the coating die, the composition becomes supersaturated and gas bubbles nucleate to form a foamed insulation. This foaming primarily occurs in the air gap between the coating die and a water trough providing cooling and solidification of the foamed insulation. Foaming levels can be adjusted by changing gas injection level, adjusting extrusion conditions and by using a movable water trough to adjust the length of air gap before the water quenching.
Various conventional additives can be added to the expandable resin composition prior to or during the mixing of the components, and prior to or at the time of the fabrication extrusion. The additives include antioxidants, ultraviolet absorbers or stabilizers, antistatic agents, pigments, dyes, nucleating agents, reinforcing fillers or polymer additives, resistivity modifiers such as carbon black, slip agents, plasticizers, processing aids, lubricants, viscosity control agents, tackifiers, anti-blocking agents, surfactants, extender oils, metal deactivators, voltage stabilizers, flame retardant fillers and additives, crosslinking boosters and catalysts, and smoke suppressants. Blowing agents are added where chemical foaming is desired. Additives can be added in amounts ranging from less than about 0.1 to more than about 5 parts by weight for each 100 parts by weight of the resin. Fillers are generally added in larger amounts up to 200 parts by weight or more.
Examples of antioxidants are: hindered phenols such as tetrakis[methylene(3,5-di-tert- butyl-4-hydroxyhydrocinnamate)]methane, bis[(beta-(3,5-ditert-butyl-4-hydroxybenzyl) methylcarboxyethyl)]sulphide, 4,4xe2x80x2-thiobis(2-methyl-6-tert-butylphenol), 4,4xe2x80x2-thiobis(2-tert-butyl-5-methylphenol), 2,2xe2x80x2-thiobis(4-methyl-6-tert-butylphenol), and thiodiethylene bis(3,5-di-tert-butyl-4-hydroxy)hydrocinnamate; phosphites and phosphonites such as tris(2,4-di-tert-butylphenyl)phosphite and di-tert-butylphenyl phosphonite; thio compounds such as dilaurylthiodipropionate, dimyristylthiodipropionate, and distearylthiodipropionate; various siloxanes; various amines such as polymerized 2,2,4-trimethyl-1,2-dihydroquinoline; and butylated hydroxytoluene. Antioxidants can be used in amounts of about 0.1 to about 5 parts by weight per 100 parts by weight of resin.
The expandable resin composition can be fluxed and mixed and the cable coated with expanded resin can be prepared in various types of extruders, some of which are described in U.S. Pat. Nos. 4,814,135; 4,857,600; 5,076,988; and 5,153,382. All types of single screw and twin screw extruders and polymer melt pumps and extrusion processes will generally be suitable in effecting the process of this invention as long as they can be adapted for extruding a foamed coating. An L arrangement of two extruders or a single screw extruder of L/D 30:1 to 35:1 are particularly mentioned. L/D is the ratio of length to diameter. While the expensive L arrangement of two extruders can be used, it is an advantage of the present invention that good dispersability can be achieved with a low cost single screw extruder at L/D 30:1 to 35:1. Other advantages of this extruder are good foaming; uniform cellular structure; good electrostatic tangent; and good electrostatic capacity. This extruder typically has a cross-head having a nipple and die, and a core electric conductor driver.
A typical extruder, commonly referred to as a fabrication extruder will have a solids feed hopper at its upstream end and a melt forming die at its downstream end. The hopper feeds unfluxed plastics into the feed section of a barrel containing the processing screw(s) that flux and ultimately pump the plastic melt through the forming die. At the downstream end, between the end of the screw and the die, there is often a melt screen/breaker plate assembly to remove contaminants. Fabrication extruders typically accomplish the mechanisms of solids conveying and compression, plastics fluxing, melt mixing and melt pumping although some two stage configurations use a separate melt fed extruder or melt pump equipment for the melt pumping mechanism. Extruder barrels are equipped with barrel heating and cooling features for startup and improved steady state temperature control. Modern equipment usually incorporates multiple heating/cooling zones starting at the rear feed zone and segmenting the barrel and downstream shaping die. The L/D of each barrel can be in the range of about 25:1 to about 35:1.
The expanded resin composition is useful in combination with electrical conductors comprised of metal such as copper or of carbon, or with communications media such as glass or plastic filaments used, for example in fiber optics applications. The term xe2x80x9csurroundedxe2x80x9d as it applies to a substrate such as copper wire or glass fiber being surrounded by an insulating composition, jacketing material, or other cable layer is considered to include extruding around the substrate; coating the substrate; or wrapping around the substrate as is well known by those skilled in the art.
All molecular weights are weight average molecular weights unless otherwise designated.
The patents and application mentioned in this specification are incorporated by reference herein.
The invention is illustrated by the following examples. Polymer ratios are weight ratios and percents of components are weight percents.